DEFINITION
Sodium hydroxide forms hard white, very hygroscopic crystals, melting at 322 o C.
Due to the strong corrosive effect on fabrics, skin, paper and other organic substances, it is called caustic soda. In engineering, sodium hydroxide is often referred to as caustic soda.
In water, sodium hydroxide dissolves with the release of a large amount of heat due to the formation of hydrates.
Sodium hydroxide should be stored in well-closed containers, as it easily absorbs carbon dioxide from the air, gradually turning into sodium carbonate.
Rice. 1. Sodium hydroxide. Appearance.
Obtaining sodium hydroxide
The main method for obtaining sodium hydroxide is the electrolysis of an aqueous solution of sodium chloride. During electrolysis, hydrogen ions are discharged at the cathode and simultaneously sodium ions and hydroxide ions accumulate near the cathode, i.e. sodium hydroxide is obtained; chlorine is released at the anode.
2NaCl + 2H 2 O \u003d H 2 + Cl 2 + 2NaOH.
In addition to the electrolytic method for obtaining sodium hydroxide, sometimes the older method is also used - boiling a solution of soda with slaked lime:
Chemical properties of sodium hydroxide
Sodium hydroxide reacts with acids to form salts and water (neutralization reaction):
NaOH + HCl \u003d NaCl + H 2 O;
2NaOH + H 2 SO 4 \u003d Na 2 SO 4 + H 2 O.
A solution of sodium hydroxide changes the color of indicators, so, for example, when litmus, phenolphthalein or methyl orange are added to a solution of this alkali, their color will turn blue, crimson and yellow, respectively.
Sodium hydroxide reacts with salt solutions (if they contain a metal capable of forming an insoluble base) and acid oxides:
Fe 2 (SO 4) 3 + 6NaOH \u003d 2Fe (OH) 3 ↓ + 3Na 2 SO 4;
2NaOH + CO 2 \u003d Na 2 CO 3 + H 2 O.
Application of sodium hydroxide
Sodium hydroxide is one of the most important products of the basic chemical industry. In large quantities, it is consumed to purify oil refinery products; sodium hydroxide is widely used in soap, paper, textile and other industries, as well as in the production of artificial fibers.
Examples of problem solving
EXAMPLE 1
| The task | Calculate the mass of sodium hydroxide that can react with a 300 ml concentrated hydrochloric acid solution (HCl mass fraction 34%, density 1.168 kg/l). |
| Solution | Let's write the reaction equation: NaOH + HCl \u003d NaCl + H 2 O. Let's find the mass of the hydrochloric acid solution, as well as the mass of the dissolved substance HCl in it: m solution = V solution × ρ; m solution \u003d 0.3 × 1.168 \u003d 0.3504 kg \u003d 350.4 g. ω = msolute / msolution × 100%; msolute = ω / 100% ×m solution ; msolute (HCl) = ω (HCl) / 100% ×m solution ; msolute (HCl) = 34 / 100% × 350.4 = 11.91 g. Calculate the number of moles of hydrochloric acid (molar mass is 36.5 g / mol): n(HCl) = m(HCl) / M(HCl); n (HCl) = 11.91 / 36.5 = 0.34 mol. According to the reaction equation n (HCl) :n (NaOH) = 1: 1. Hence, n (NaOH) \u003d n (HCl) \u003d 0.34 mol. Then the mass of sodium hydroxide that has entered into the reaction will be equal to (molar mass - 40 g / mol): m (NaOH) = n (NaOH) × M (NaOH); m (NaOH) \u003d 0.34 × 40 \u003d 13.6 g. |
| Answer | The mass of sodium hydroxide is 13.6 g. |
EXAMPLE 2
| The task | Calculate the mass of sodium carbonate that will be required to obtain sodium hydroxide by reaction with calcium hydroxide weighing 3.5 g. |
| Solution | Let us write the reaction equation for the interaction of sodium carbonate with calcium hydroxide to form sodium hydroxide: Na 2 CO 3 + Ca (OH) 2 \u003d CaCO 3 ↓ + 2NaOH. Calculate the amount of calcium hydroxide substance (molar mass - 74 g / mol): n (Ca (OH) 2) \u003d m (Ca (OH) 2) / M (Ca (OH) 2); n (Ca (OH) 2) \u003d 3.5 / 74 \u003d 0.05 mol. According to the reaction equation n (Ca (OH) 2): n (Na 2 CO 3) \u003d 1: 1. Then the number of moles of sodium carbonate will be equal to: n (Na 2 CO 3) \u003d n (Ca (OH) 2) \u003d 0.05 mol. Find the mass of sodium carbonate (molar mass - 106 g / mol): m (Na 2 CO 3) \u003d n (Na 2 CO 3) × M (Na 2 CO 3); m (Na 2 CO 3) \u003d 0.05 × 106 \u003d 5.3 g. |
| Answer | The mass of sodium carbonate is 5.3 g. |
· Precautions for handling sodium hydroxide · Literature ·
Sodium hydroxide can be produced industrially by chemical and electrochemical methods.
Chemical methods for obtaining sodium hydroxide
Chemical methods for producing sodium hydroxide include calcareous and ferritic.
Chemical methods for producing sodium hydroxide have significant drawbacks: a lot of energy carriers are consumed, the resulting caustic soda is heavily contaminated with impurities.
Today, these methods have been almost completely superseded by electrochemical manufacturing methods.
lime method
The lime method for producing sodium hydroxide consists in the interaction of a soda solution with slaked lime at a temperature of about 80 ° C. This process is called caustication; it goes through the reaction:
Na 2 CO 3 + Ca (OH) 2 \u003d 2NaOH + CaCO 3
As a result of the reaction, a solution of sodium hydroxide and a precipitate of calcium carbonate are obtained. Calcium carbonate is separated from the solution, which is evaporated to obtain a molten product containing about 92% of the mass. NaOH. After NaOH is melted and poured into iron drums, where it solidifies.
ferrite method
The ferritic method for producing sodium hydroxide consists of two stages:
- Na 2 CO 3 + Fe 2 O 3 \u003d 2NaFeO 2 + CO 2
- 2NaFeO 2 + xH 2 O \u003d 2NaOH + Fe 2 O 3 * xH 2 O
Reaction 1 is the process of sintering soda ash with iron oxide at a temperature of 1100-1200 °C. In addition, sodium speck is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction 2; a solution of sodium hydroxide and a precipitate of Fe 2 O 3 *xH 2 O are obtained, which, after separating it from the solution, is returned to the process. The resulting alkali solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% of the mass. NaOH, and then get a solid product in the form of granules or flakes.
Electrochemical methods for producing sodium hydroxide
Electrochemically sodium hydroxide is obtained electrolysis of halite solutions(a mineral consisting mainly of table salt NaCl) with the simultaneous production of hydrogen and chlorine. This process can be represented by the summary formula:
2NaCl + 2H 2 O ± 2e - → H 2 + Cl 2 + 2NaOHCaustic alkali and chlorine are produced by three electrochemical methods. Two of them are electrolysis with a solid cathode (diaphragm and membrane methods), the third is electrolysis with a liquid mercury cathode (mercury method).
All three methods of obtaining chlorine and caustic are used in world production practice, with a clear trend towards an increase in the share of membrane electrolysis.
In Russia, approximately 35% of all caustic produced is produced by electrolysis with a mercury cathode and 65% by electrolysis with a solid cathode.
diaphragm method
Scheme of an old diaphragm electrolytic cell for the production of chlorine and lye: BUT- anode, IN- insulators, FROM- cathode, D- space filled with gases (above the anode - chlorine, above the cathode - hydrogen), M- diaphragm
The simplest of the electrochemical methods, in terms of organizing the process and structural materials for the electrolyzer, is the diaphragm method for producing sodium hydroxide.
The salt solution in the diaphragm electrolytic cell is continuously fed into the anode space and flows through an asbestos diaphragm, usually deposited on a steel cathode grid, to which, in some cases, a small amount of polymer fibers is added.
In many designs of electrolyzers, the cathode is completely immersed under the anolyte layer (electrolyte from the anode space), and the hydrogen released on the cathode grid is removed from under the cathode using gas pipes, without penetrating through the diaphragm into the anode space due to countercurrent.
Counterflow is a very important feature of the diaphragm cell design. It is thanks to the countercurrent flow directed from the anode space to the cathode space through a porous diaphragm that it becomes possible to separately obtain lye and chlorine. The countercurrent flow is designed to counteract the diffusion and migration of OH - ions into the anode space. If the amount of countercurrent is insufficient, then hypochlorite ion (ClO -) begins to form in the anode space in large quantities, which, after that, can be oxidized at the anode to the chlorate ion ClO 3 - . The formation of chlorate ion seriously reduces the current efficiency of chlorine and is a major side process in this method of producing sodium hydroxide. The release of oxygen is also harmful, which, moreover, leads to the destruction of the anodes and, if they are made of carbon materials, to the ingress of phosgene impurities into chlorine.
Anode: 2Cl - 2e → Cl 2 - main process 2H 2 O - 2e - → O 2 + 4H + Cathode: 2H 2 O + 2e → H 2 + 2OH - main process ClO - + H 2 O + 2e - → Cl - + 2OH - ClO 3 - + 3H 2 O + 6e - → Cl - + 6OH -
Graphite or carbon electrodes can be used as an anode in diaphragm electrolyzers. To date, they have mainly been replaced by titanium anodes with a ruthenium oxide-titanium coating (ORTA anodes) or other low-consumption anodes.
At the next stage, the electrolytic liquor is evaporated and the content of NaOH in it is adjusted to a commercial concentration of 42-50 wt%. in accordance with the standard.
Table salt, sodium sulfate and other impurities, when their concentration in solution increases above their solubility limit, precipitate. The caustic solution is decanted from the sediment and transferred as a finished product to the warehouse or the evaporation stage is continued to obtain a solid product, followed by melting, flaking or granulation.
The reverse, that is, table salt crystallized into a precipitate, is returned back to the process, preparing the so-called reverse brine from it. From it, in order to avoid the accumulation of impurities in solutions, impurities are separated before preparing the return brine.
The loss of anolyte is replenished by adding fresh brine obtained by underground leaching of salt layers, mineral brines such as bischofite, previously purified from impurities, or by dissolving halite. Before mixing it with the reverse brine, fresh brine is cleaned of mechanical suspensions and a significant part of calcium and magnesium ions.
The resulting chlorine is separated from water vapor, compressed and fed either to the production of chlorine-containing products or to liquefaction.
Due to the relative simplicity and low cost, the diaphragm method for producing sodium hydroxide is still widely used in industry.
Membrane method
The membrane method for the production of sodium hydroxide is the most energy efficient, but at the same time it is difficult to organize and operate.
From the point of view of electrochemical processes, the membrane method is similar to the diaphragm method, but the anode and cathode spaces are completely separated by an anion-impermeable cation-exchange membrane. Thanks to this property, it becomes possible to obtain purer liquors than in the case of the diaphragm method. Therefore, in a membrane electrolyzer, in contrast to a diaphragm cell, there is not one stream, but two.
As in the diaphragm method, a salt solution flow enters the anode space. And in the cathode - deionized water. A stream of depleted anolyte flows out of the cathode space, which also contains impurities of hypochlorite and chlorate ions and chlorine, and from the anode space - lye and hydrogen, which practically do not contain impurities and are close to commercial concentration, which reduces energy costs for their evaporation and purification.
The alkali produced by membrane electrolysis is almost as good as that produced by the mercury cathode method and is slowly replacing the alkali produced by the mercury method.
At the same time, the feeding solution of salt (both fresh and recycled) and water are preliminarily cleaned of any impurities as much as possible. Such thorough cleaning is determined by the high cost of polymeric cation exchange membranes and their vulnerability to impurities in the feed solution.
In addition, the limited geometric shape and, in addition, the low mechanical strength and thermal stability of ion-exchange membranes largely determine the relatively complex designs of membrane electrolysis plants. For the same reason, membrane plants require the most complex automatic control and management systems.
Scheme of a membrane electrolyzer.Mercury method with liquid cathode
Among the electrochemical methods for producing alkalis, the most effective method is electrolysis with a mercury cathode. Alkalis obtained by electrolysis with a liquid mercury cathode are much cleaner than those obtained by the diaphragm method (this is critical for some industries). For example, in the production of artificial fibers, only high-purity caustic can be used), and in comparison with the membrane method, the organization of the process for obtaining alkali by the mercury method is much simpler.
Scheme of a mercury electrolyzer.The installation for mercury electrolysis consists of an electrolyser, an amalgam decomposer and a mercury pump, interconnected by mercury-conducting communications.
The cathode of the electrolyzer is a flow of mercury pumped by a pump. Anodes - graphite, carbon or low-wear (ORTA, TDMA or others). Together with mercury, a stream of feeding table salt continuously flows through the electrolyzer.
At the anode, chlorine ions are oxidized from the electrolyte, and chlorine is released:
2Cl - 2e → Cl 2 0 - main process 2H 2 O - 2e - → O 2 + 4H + 6ClO - + 3H 2 O - 6e - → 2ClO 3 - + 4Cl - + 1.5O 2 + 6H +Chlorine and anolyte are removed from the electrolyzer. The anolyte leaving the electrolyzer is saturated with fresh halite, the impurities introduced with it are removed from it, and, in addition, washed out from the anodes and structural materials, and returned to electrolysis. Before saturation, the chlorine dissolved in it is extracted from the anolyte.
Sodium ions are reduced at the cathode, which form a weak solution of sodium in mercury (sodium amalgam):
Na + + e \u003d Na 0 nNa + + nHg = Na + HgThe amalgam continuously flows from the electrolyser to the amalgam decomposer. The decomposer is also continuously fed with highly purified water. In it, sodium amalgam, as a result of a spontaneous chemical process, is almost completely decomposed by water with the formation of mercury, a caustic solution and hydrogen:
Na + Hg + H 2 O = NaOH + 1/2H 2 + HgThe caustic solution obtained in this way, which is a commercial product, contains practically no impurities. Mercury is almost completely freed from sodium and returned to the electrolyzer. Hydrogen is removed for purification.
However, complete purification of the alkali solution from mercury residues is practically impossible, therefore this method is associated with leakage of metallic mercury and its vapors.
The growing requirements for environmental safety of production and the high cost of metallic mercury lead to the gradual replacement of the mercury method by methods of producing alkali with a solid cathode, especially the membrane method.
Laboratory methods of obtaining
In the laboratory, sodium hydroxide is sometimes produced by chemical means, but more commonly a small diaphragm or membrane-type electrolyzer is used.
Introduction .
Sodium hydroxide or caustic soda (NaOH), chlorine, hydrochloric acid HC1 and hydrogen are currently produced in industry by electrolysis of a sodium chloride solution.
Caustic soda or sodium hydroxide - a strong alkali, called caustic soda in everyday life, is used in soap making, in the production of alumina - an intermediate product for obtaining metallic aluminum, in the paint and varnish, oil refining industries, in the production of rayon, in the organic synthesis industry and other sectors of the national economy.
When working with chlorine, hydrogen chloride, hydrochloric acid and caustic soda, it is necessary to strictly follow the safety rules: inhalation of chlorine causes a sharp cough and suffocation, inflammation of the mucous membranes of the respiratory tract, pulmonary edema, and later the formation of inflammatory foci in the lungs.
Hydrogen chloride, even at low levels in the air, causes irritation in the nose and larynx, tingling in the chest, hoarseness and suffocation. In chronic poisoning with low concentrations, teeth are especially affected, the enamel of which is rapidly destroyed.
Hydrochloric acid poisoning is very similar from chlorine poisoning.
Chemical methods for producing sodium hydroxide.
Chemical methods for producing sodium hydroxide include calcareous and ferritic.
The lime method for producing sodium hydroxide consists in the interaction of a soda solution with milk of lime at a temperature of about 80 ° C. This process is called caustication; it is described by the reaction
Na 2 C0 3 + Ca (OH) 2 \u003d 2NaOH + CaC0 3 (1)
solution-settlement
According to reaction (1), a solution of sodium hydroxide and a precipitate of calcium carbonate are obtained. The calcium carbonate is separated from the solution, which is evaporated to obtain a molten product containing about 92% NaOH. Molten NaOH is poured into iron drums where it solidifies.
The ferritic method is described by two reactions:
Na 2 C0 3 + Fe 2 0 3 = Na 2 0 Fe 2 0 3 + C0 2 (2)
sodium ferrite
Na 2 0 Fe 2 0 3 -f H 2 0 \u003d 2 NaOH + Fe 2 O 3 (3)
solution precipitate
reaction (2) shows the process of sintering soda ash with iron oxide at a temperature of 1100-1200°C. In this case, speck - ferrite sodium is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction (3); a solution of sodium hydroxide and a precipitate of Fe 2 O 3 are obtained, which, after separating it from the solution, is returned to the process. The solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% NaOH.
Chemical methods for producing sodium hydroxide have significant drawbacks: a large amount of fuel is consumed, the resulting caustic soda is contaminated with impurities, maintenance of the apparatus is laborious, etc. At present, these methods have been almost completely replaced by the electrochemical method of production.
The concept of electrolysis and electrochemical processes.
Electrochemical processes are called chemical processes occurring in aqueous solutions or melts under the influence of a direct electric current.
Solutions and melts of salts, solutions of acids and alkalis, called electrolytes, are conductors of the second kind, in which the transfer of electric current is carried out by ions. (In conductors of the first kind, such as metals, the current is carried by electrons.) When an electric current passes through the electrolyte, ions are discharged at the electrodes and the corresponding substances are released. This process is called electrolysis. The apparatus in which electrolysis is carried out is called an electrolyzer or electrolytic bath.
Electrolysis is used to obtain a number of chemical products - chlorine, hydrogen, oxygen, alkalis, etc. It should be noted that electrolysis produces high-purity chemical products, in some cases unattainable with chemical methods of their production.
The disadvantages of electrochemical processes include high energy consumption during electrolysis, which increases the cost of the products obtained. In this regard, it is advisable to carry out electrochemical processes only on the basis of cheap electrical energy.
Raw material for the production of sodium hydroxide.
For the production of sodium hydroxide, chlorine, hydrogen, a solution of common salt is used, which is subjected to electrolysis. Table salt occurs in nature in the form of underground deposits of rock salt, in the waters of lakes and seas, and in the form of natural brines or solutions. Rock salt deposits are located in the Donbass, the Urals, Siberia, Transcaucasia and other regions. Rich in salt in our country and some lakes.
In the summer, water evaporates from the surface of the lakes, and table salt falls out in the form of crystals. Such salt is called self-planting. Sea water contains up to 35 g/l of sodium chloride. In places with a hot climate, where intense evaporation of water occurs, concentrated solutions of sodium chloride are formed, from which it crystallizes. In the bowels of the earth, in the salt layers, underground waters flow, which dissolve NaCl and form underground brines that come out through boreholes to the surface.
Salt solutions, regardless of the way they are obtained, contain impurities of calcium and magnesium salts, and before they are transferred to the electrolysis workshops, they are purified from these salts. Purification is necessary because in the process of electrolysis, poorly soluble calcium and magnesium hydroxides can be formed, which disrupt the normal course of electrolysis.
Cleaning of brines is carried out with a solution of soda and lime milk. In addition to chemical purification, solutions are freed from mechanical impurities by sedimentation and filtration.
The electrolysis of common salt solutions is carried out in baths with a solid iron (steel) cathode and with diaphragms and in baths with a liquid mercury cathode. In any case, industrial electrolyzers used for the equipment of modern large chlorine plants must have high productivity, simple design, be compact, work reliably and stably.
Electrolysis of sodium chloride solutions in baths with a steel cathode and a graphite anode .
It makes it possible to obtain sodium hydroxide, chlorine and hydrogen in one apparatus (electrolyzer). When passing a direct electric current through an aqueous solution of sodium chloride, one can expect the release of chlorine:
2CI - - 2ndÞ С1 2 (a)
as well as oxygen:
20N - - 2ndÞ 1/2O 2 + H 2 O (b)
H 2 0-2eÞ1 / 2О 2 + 2H +
The normal electrode potential of the discharge of OH - -ions is + 0.41 in, and the normal electrode potential of the discharge of chlorine ions is + 1.36 in. In a neutral saturated solution of sodium chloride, the concentration of hydroxyl ions is about 1 10 - 7 g-eq/l. At 25°C, the equilibrium potential for the discharge of hydroxide ions will be
Equilibrium potential of the discharge, chloride ions at a concentration of NaCI in a solution of 4.6 g-eq/l equals
Therefore, at the anode with a small overvoltage, oxygen should be discharged first.
However, on graphite anodes, the oxygen overvoltage is much higher than the chlorine overvoltage, and therefore, they will mainly discharge C1 - ions with the release of gaseous chlorine according to reaction (a).
The release of chlorine is facilitated with an increase in the concentration of NaCl in the solution due to a decrease in the value of the equilibrium potential. This is one of the reasons for the use of concentrated sodium chloride solutions containing 310-315 g/l.
At the cathode in an alkaline solution, water molecules are discharged according to the equation
H 2 0 + e \u003d H + OH - (c)
Hydrogen atoms after recombination are released in the form of molecular hydrogen
2H Þ H 2 (g)
The discharge of sodium ions from aqueous solutions on a solid cathode is impossible due to the higher potential of their discharge compared to hydrogen. Therefore, the hydroxide ions remaining in the solution form an alkali solution with sodium ions.
The decomposition process of NaCl can be expressed in this way by the following reactions:
i.e., chlorine is formed at the anode, and hydrogen and sodium hydroxide are formed at the cathode.
During electrolysis, along with the main described processes, side processes can also occur, one of which is described by equation (b). In addition, the chlorine released at the anode is partially dissolved in the electrolyte and hydrolyzed by the reaction
In the case of diffusion of alkali (OH - ions) to the anode or displacement of cathode and anode products, hypochlorous and hydrochloric acids are neutralized with alkali to form hypochlorite and sodium chloride:
NOS1 + NaOH \u003d NaOCl + H 2 0
HC1 + NaOH \u003d NaCl + H 2 0
Ions ClO - on the anode are easily oxidized to ClO 3 - . Therefore, hypochlorite, sodium chloride and sodium chlorate will be generated due to side processes during electrolysis, which will lead to a decrease in current efficiency and energy efficiency. In an alkaline environment, the release of oxygen at the anode is facilitated, which will also worsen the electrolysis performance.
To reduce the occurrence of side reactions, it is necessary to create conditions that prevent the mixing of cathode and anode products. These include the separation of the cathode and anode spaces by a diaphragm and the filtration of the electrolyte through the diaphragm in the direction opposite to the movement of OH - ions to the anode. Such diaphragms are called filter diaphragms and are made of asbestos.
Introduction
You came to the store looking to buy unscented soap. Naturally, in order to understand which products from this range have a smell and which do not, you pick up each bottle of soap and read its composition and properties. Finally, they chose the right one, but while looking at the various compositions of the soap, they noticed a strange trend - on almost all the bottles it was written: "Soap contains sodium hydroxide in the structure." This is the standard history of most people's acquaintance with sodium hydroxide. Some half of the people will "spit and forget", and some will want to know more about him. So for them today I will tell you what kind of substance it is.
Definition
Sodium hydroxide (formula NaOH) is the most common alkali in the world. For reference: alkali is a base that is highly soluble in water.
Name
In various sources, it can be called sodium hydroxide, caustic soda, caustic, caustic soda or caustic alkali. Although the name "caustic alkali" can be applied to all substances in this group. Only in the XVIII century they were given separate names. There is also an "inverted" name of the substance described now - sodium hydroxide, usually used in Ukrainian translations.
Properties
As I said, sodium hydroxide is highly soluble in water. If you put even a small piece of it in a glass of water, after a few seconds it will ignite and will “rush” and “jump” along its surface with a hiss (photo). And this will continue until he completely dissolves in it. If, after the reaction is complete, you dip your hand into the resulting solution, it will be soapy to the touch. To find out how strong the alkali is, indicators are lowered into it - phenolphthalein or methyl orange. Phenolphthalein in it acquires a crimson color, and methyl orange - yellow. Sodium hydroxide, like all alkalis, contains hydroxide ions. The more of them in the solution, the brighter the color of the indicators and the stronger the alkali.
Receipt
There are two ways to obtain sodium hydroxide: chemical and electrochemical. Let's consider each of them in more detail.
Application
The delignification of cellulose, the production of cardboard, paper, fibreboard and artificial fibers cannot do without sodium hydroxide. And when it reacts with fats, soap, shampoos and other detergents are obtained. In chemistry, it is used as a reactant or catalyst in many reactions. Sodium hydroxide is also known as food additive E524. And this is not all areas of its application.
Conclusion
Now you know everything about sodium hydroxide. As you can see, it brings a lot of benefits to a person - both in industry and in everyday life.
Chemical methods for producing sodium hydroxide include calcareous and ferritic.
Chemical methods for producing sodium hydroxide have significant drawbacks: a lot of energy carriers are consumed, the resulting caustic soda is heavily contaminated with impurities.
Today, these methods have been almost completely superseded by electrochemical manufacturing methods.
lime method
The lime method for producing sodium hydroxide consists in the interaction of a soda solution with slaked lime at a temperature of about 80 ° C. This process is called caustication; it goes through the reaction:
Na 2 SO 3 + Ca(OH) 2 = 2NaOH + CaCO 3
As a result of the reaction, a solution of sodium hydroxide and a precipitate of calcium carbonate are obtained. Calcium carbonate is separated from the solution, which is evaporated to obtain a molten product containing about 92% of the mass. NaOH. After NaOH is melted and poured into iron drums, where it solidifies.
ferrite method
The ferritic method for producing sodium hydroxide consists of two stages:
Na 2 SO 3 + Fe 2 ABOUT 3 = 2NaFeO 2 + CO 2
2NaFeО 2 +xH 2 O = 2NaOH + Fe 2 O 3 *xH 2 ABOUT
Reaction 1 is the process of sintering soda ash with iron oxide at a temperature of 1100-1200 °C. In addition, sodium speck is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction 2; a solution of sodium hydroxide and a precipitate of Fe 2 O 3 *xH 2 O are obtained, which, after separating it from the solution, is returned to the process. The resulting alkali solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% of the mass. NaOH, and then get a solid product in the form of granules or flakes.
Electrochemical methods for producing sodium hydroxide
Electrochemically sodium hydroxide is obtained electrolysis of halite solutions(a mineral consisting mainly of table salt NaCl) with the simultaneous production of hydrogen and chlorine. This process can be represented by the summary formula:
2NaCl + 2H 2 About ±2e - → H 2 +Cl 2 + 2NaOH
Caustic alkali and chlorine are produced by three electrochemical methods. Two of them are electrolysis with a solid cathode (diaphragm and membrane methods), the third is electrolysis with a liquid mercury cathode (mercury method).
All three methods of obtaining chlorine and caustic are used in world production practice, with a clear trend towards an increase in the share of membrane electrolysis.
7. Purification of sulfur dioxide from catalytic poisons.
Gaseous emissions have a very unfavorable effect on the ecological situation in the locations of these industrial enterprises, and also worsen the sanitary and hygienic working conditions. Aggressive mass emissions include nitrogen oxides, hydrogen sulfide, sulfur dioxide, carbon dioxide and many other gases.
For example, nitric acid, sulfuric acid and other plants in our country annually emit tens of millions of cubic meters of nitrogen oxides into the atmosphere, which are a strong and dangerous poison. From these oxides of nitrogen, thousands of tons of nitric acid could be produced.
An equally important task is the purification of gases from sulfur dioxide. The total amount of sulfur that is emitted into the atmosphere in our country only in the form of sulfur dioxide is about 16 million tons . in year. From this amount of sulfur, up to 40 million tons of sulfuric acid can be produced.
A significant amount of sulfur, mainly in the form of hydrogen sulfide, is contained in coke oven gas.
With flue gases from factory pipes and power plants, several billion cubic meters of carbon dioxide are released into the atmosphere every year. This gas can be used to produce efficient carbonaceous fertilizers.
The given examples show what huge material values are emitted into the atmosphere with gaseous emissions.
But these emissions cause more serious damage by poisoning the air in cities and enterprises: poisonous gases destroy vegetation, have an extremely harmful effect on human and animal health, destroy metal structures and corrode equipment.
Although domestic industrial enterprises have not been operating at full capacity in recent years, the problem of combating harmful emissions is very acute. And taking into account the general ecological situation on the planet, it is necessary to take the most urgent and most radical measures to purify exhaust gases from harmful impurities.
Catalytic poisons
contact poisons, substances that cause "poisoning" of catalysts (See. Catalysts) (usually heterogeneous), i.e., reducing their catalytic activity or completely stopping the catalytic action. Poisoning of heterogeneous catalysts occurs as a result of the adsorption of a poison or its chemical transformation product on the catalyst surface. Poisoning can be reversible or irreversible. Thus, in the reaction of ammonia synthesis on an iron catalyst, oxygen and its compounds poison Fe reversibly; in this case, when exposed to a pure mixture of N 2 + H 2, the surface of the catalyst is freed from oxygen and poisoning is reduced. Sulfur compounds poison Fe irreversibly; the action of a pure mixture fails to restore the activity of the catalyst. To prevent poisoning, the reaction mixture fed to the catalyst is thoroughly purified. Among the most common K. I. metal catalysts include substances containing oxygen (H 2 O, CO, CO 2), sulfur (H 2 S, CS 2, C 2 H 2 SH, etc.), Se, Te, N, P, As, Sb, as well as unsaturated hydrocarbons (C 2 H 4 , C 2 H 2) and metal ions (Cu 2+ , Sn 2+ , Hg 2+ , Fe 2+ , Co 2+ , Ni 2+). Acid catalysts are usually poisoned by base impurities, while basic catalysts are poisoned by acid impurities.
8. Obtaining nitrous gases.
The nitrogen oxides released after bleaching are condensed in water and brine condensers and used to prepare the raw mixture. Since the boiling point of N 2 O 4 is 20.6 ° C at a pressure of 0.1 MPa, under these conditions, gaseous NO 2 can be completely condensed (saturated vapor pressure of N 2 O 4 at 21.5 ° C over liquid N 2 O 4 equal to 0.098 MPa, i.e. less than atmospheric). Another way to obtain liquid nitrogen oxides is to condense them under pressure and at low temperature. If we recall that during the contact oxidation of NH 3 at atmospheric pressure, the concentration of nitrogen oxides is not more than 11% vol., Their partial pressure corresponds to 83.5 mm Hg. The pressure of nitrogen oxides above the liquid (vapor pressure) at the condensation temperature (–10 °C) is 152 mm Hg. This means that without increasing the condensation pressure, liquid nitrogen oxides cannot be obtained from these gases, therefore, the condensation of nitrogen oxides from such a nitrous gas at a temperature of –10 ° C begins at a pressure of 0.327 MPa. The degree of condensation increases sharply with an increase in pressure up to 1.96 MPa, with a further increase in pressure, the degree of condensation changes slightly.
The processing of nitrous gas (i.e., after the conversion of NH 3) into liquid nitrogen oxides is ineffective, because even at Р=2.94 MPa, the degree of condensation is 68.3%.
In conditions of condensation of pure N 2 O 4, cooling should not be carried out below a temperature of -10 ° C, because at –10.8 °С N 2 O 4 crystallizes. The presence of impurities NO, NO 2 , H 2 O reduces the crystallization temperature. So a mixture having the composition N 2 O 4 + 5% N 2 O 3 crystallizes at -15.8 ° С.
The resulting liquid nitrogen oxides are stored in steel tanks.
9. Obtaining simple and double superphosphate
"Superphosphate" - a mixture of Ca (H 2 PO 4) 2 * H 2 O and CaSO 4. The most common simple mineral phosphorus fertilizer. Phosphorus in superphosphate is present mainly in the form of monocalcium phosphate and free phosphoric acid. The fertilizer contains gypsum and other impurities (iron and aluminum phosphates, silica, fluorine compounds, etc.). Simple superphosphate is obtained from phosphorites by treating with sulfuric acid, according to the reaction:
Sa 3 (RO 4 ) 2 + 2H 2 SO 4 = Sa(H 2 PO 4 ) 2 + 2CaSO 4 .
Simple superphosphate- gray powder, almost non-caking, moderately dispersible; in fertilizer 14-19.5% P 2 O 5 digestible by plants. The essence of the production of simple superphosphate is the conversion of natural fluorapatite, insoluble in water and soil solutions, into soluble compounds, mainly into Ca(H 2 PO 4) 2 monocalcium phosphate. The decomposition process can be represented by the following summary equation:
2Ca 5 F (PO 4) 3 + 7H 2 SO 4 + 3H 2 O \u003d 3Ca (H 2 PO 4) 2 * H 2 O] + 7 + 2HF; (1) ΔН= - 227.4 kJ.
In practice, during the production of simple superphosphate, decomposition proceeds in two stages. In the first stage, about 70% of apatite reacts with sulfuric acid. This produces phosphoric acid and calcium sulfate hemihydrate:
Ca 5 F (PO 4) 3 + 5H 2 SO 4 + 2.5H 2 O \u003d 5 (CaSO 4 * 0.5H 2 O) + 3H3PO 4 + HF (2)
The functional scheme for obtaining simple superphosphate is shown in fig. The main processes take place in the first three stages: mixing of raw materials, formation and solidification of superphosphate pulp, ripening of superphosphate in a warehouse.
Rice. Functional diagram of the production of simple superphosphate
To obtain a commercial product of a higher quality, superphosphate after ripening is subjected to neutralization with solid additives (limestone, phosphate rock, etc.) and granulated.
Double superphosphate- concentrated phosphate fertilizer. The main phosphorus-containing component is calcium dihydroorthophosphate monohydrate Ca (H 2 PO 4) 2 H 2 O. It usually also contains other calcium and magnesium phosphates. Compared to simple phosphate, it does not contain ballast - CaSO 4 . The main advantage of double superphosphate is a small amount of ballast, that is, it reduces transport costs, storage costs, packaging
Double superphosphate is produced by the action of sulfuric acid H 2 SO 4 on natural phosphates. In Russia, the flow method is mainly used: the decomposition of raw materials, followed by granulation and drying of the resulting pulp in a drum granulator-dryer. Commercial double superphosphate from the surface is neutralized with chalk or NH 3 to obtain a standard product. A certain amount of double superphosphate is produced in a chamber way. Phosphorus-containing components are basically the same as in simple superphosphate, but in larger quantities, and the content of CaSO 4 is 3-5%. When heated above 135-140 °C, double superphosphate begins to decompose and melt in water of crystallization, after cooling it becomes porous and brittle. At 280-320 °C, orthophosphates turn into meta-, pyro- and polyphosphates, which are in digestible and partially water-soluble forms. It melts at 980 °C, turning after cooling into a glassy product, in which 60-70% of the metaphosphates are citrate-soluble. Double Superphosphate contains 43-49% of assimilable phosphoric anhydride (phosphorus pentoxide) P 2 O 5 (37-43% water-soluble), 3.5-6.5% free phosphoric acid H 3 PO 4 (2.5-4.6% R 2 O 5):
Ca 3 (PO 4) 2 + 2H 2 SO 4 \u003d Ca (H 2 PO 4) 2 + 2CaSO 4
There is also a method for the decomposition of phosphorus-containing raw materials with phosphoric acid:
Ca 5 (PO 4) 3 F + 7H 3 PO 4 \u003d 5Ca (H 2 PO 4) 2 + HF
Block diagram of the technological process for the production of double superphosphate: 1 - mixing of crushed phosphorite and phosphoric acid; 2 - decomposition of phosphorite of the 1st stage; 3 - decomposition of phosphorite II stage; 4 - pulp granulation; 5 - purification of phosphorus-containing gases from dust; 6 - drying of pulp granules; 7 - obtaining flue gases (in the furnace); 8 - screening of dry product; 9 - grinding of a large fraction; 10 - separation of fine and medium (commodity) fractions on the second screen; 11 - mixing of crushed large fraction and fine; 12 - ammonization (neutralization) of residual phosphoric acid; 13 - purification of gases containing ammonia and dust; 14 - cooling of the neutralized commodity fraction of double superphosphate;
10. Obtaining extraction orthophosphoric acid
Preparation of extractive phosphoric acid
Immediately before obtaining EPA, phosphorus is obtained using a special technology
Fig 1. Scheme of phosphorus production: 1 - raw material bunkers; 2 - mixer; 3 - ring feeder; 4 - charge hopper; 5 - electric furnace; 6 - ladle for slag; 7 - ladle for ferrophosphorus; 8 - electrostatic precipitator; 5 - capacitor; 10 - collection of liquid phosphorus; 11 - sump
The extraction method (allows the production of the purest phosphoric acid) includes the main stages: combustion (oxidation) of elemental phosphorus in excess air, hydration and absorption of the resulting P4O10, condensation of phosphoric acid, and capture of fog from the gas phase. There are two ways to obtain P4O10: the oxidation of P vapor (rarely used in industry) and the oxidation of liquid P in the form of droplets or films. The degree of oxidation of P under industrial conditions is determined by the temperature in the oxidation zone, the diffusion of components, and other factors. The second stage in the production of thermal phosphoric acid - P4O10 hydration - is carried out by absorption with acid (water) or by the interaction of P4O10 vapor with water vapor. Hydration (P4O10 + 6H2O4H3PO4) proceeds through the stages of formation of polyphosphoric acids. The composition and concentration of the resulting products depend on the temperature and partial pressure of water vapor.
All stages of the process are combined in one apparatus, except for the mist collection, which is always carried out in a separate apparatus. In industry, schemes of two or three main apparatuses are usually used. Depending on the principle of gas cooling, there are three methods for the production of thermal phosphoric acid: evaporative, circulation-evaporative, heat-exchange-evaporative.
Evaporative systems based on the removal of heat during the evaporation of water or dilute phosphoric acid are the simplest in hardware design. However, due to the relatively large volume of exhaust gases, the use of such systems is advisable only in installations of small unit capacity.
Circulation-evaporation systems make it possible to combine the stages of burning P, cooling the gas phase with circulating acid, and hydrating P4O10 in one apparatus. The disadvantage of the scheme is the need to cool large volumes of acid. Heat exchange and evaporation systems combine two methods of heat removal: through the wall of the combustion and cooling towers, as well as by evaporating water from the gas phase; a significant advantage of the system is the absence of acid circulation circuits with pumping and cooling equipment.
Domestic enterprises operate technological schemes with a circulation-evaporative cooling method (double-tower system). Distinctive features of the scheme: the presence of an additional tower for gas cooling, the use of efficient plate heat exchangers in the circulation circuits; the use of a high-performance burner for burning P, which ensures uniform fine atomization of the liquid P jet and its complete combustion without the formation of lower oxides.
The technological scheme of the installation with a capacity of 60 thousand tons per year of 100% H3PO4 is shown in fig. 2. Molten yellow phosphorus is atomized with heated air at a pressure of up to 700 kPa through a nozzle in a combustion tower sprayed with circulating acid. The acid heated in the tower is cooled by circulating water in plate heat exchangers. Production acid containing 73-75% H3PO4 is discharged from the circulation circuit to the storage. In addition, the cooling of gases from the combustion tower and the absorption of acid are carried out in the cooling tower (hydration), which reduces the afterbirth, the temperature load on the electrostatic precipitator and contributes to effective gas purification. Heat removal in the hydration tower is carried out by circulating 50% H3PO4 cooled in plate heat exchangers. Gases from the hydration tower after being cleaned from H3PO4 mist in a plate electrostatic precipitator are released into the atmosphere. For 1 ton of 100% H3PO4, 320 kg of P is consumed.
Rice. Fig. 2. Circulation two-tower scheme for the production of extraction H3PO4: 1 - sour water collector; 2 - storage of phosphorus; 3.9 - circulation collectors; 4.10 - submersible pumps; 5.11 - plate heat exchangers; 6 - combustion tower; 7 - phosphorus nozzle; 8 - hydration tower; 12 - electrostatic precipitator; 13 - fan.
11. Catalysts for the oxidation of sulfur dioxide to sulfuric anhydride. contacting
Sulfuric anhydride is obtained by oxidizing sulfur dioxide with atmospheric oxygen:
2SO2 + O2 ↔ 2SO3,
This is a reversible reaction.
It has long been observed that iron oxide, vanadium pentoxide, and especially finely divided platinum accelerate the oxidation reaction of sulfur dioxide to sulfuric anhydride. These substances are catalysts for the oxidation of sulfur dioxide. So, for example, at 400 ° C in the presence of platinized asbestos (ie, asbestos, on the surface of which finely crushed platinum is deposited), almost 100% of sulfur dioxide is oxidized by atmospheric oxygen into sulfuric anhydride. At a higher temperature, the yield of sulfuric anhydride decreases, as the reverse reaction is accelerated - the reaction of decomposition of sulfuric anhydride into sulfur dioxide and oxygen. At 1000°C, sulfuric anhydride decomposes almost completely into the starting materials. Thus, the main conditions for the synthesis of sulfuric anhydride are the use of catalysts and heating to a certain, not too high temperature.
The synthesis of sulfuric anhydride also requires compliance with two more conditions: sulfur dioxide must be purified from impurities that inhibit the action of catalysts; sulfur dioxide and air must be dried, as moisture reduces the yield of sulfuric anhydride.
